This invention relates to compositions in the form of tablets, containing a water-softening agent. These tablets may be embodied as detergent compositions for use in fabric washing, or as water-softening tablets, which could be used in fabric washing jointly with a composition containing detergent active, or could possibly be used in other applications, e.g. in machine dishwashing as an anti-limescale product. The invention is concerned with tablets which are intended to disintegrate, usually in less than 15 minutes, when placed in water, so that the tablets are consumed when carrying out a single washing operation.
Detergent compositions in tablet form are described, for example, in GB 911204 (Unilever), U.S. Pat. No. 3953350 (Kao), JP 60-015500A (Lion), JP 60-135497A (Lion) and JP 60-135498A (Lion); and are solid commercially in Europe. Tablets have several advantages over powdered products: they do not require measuring and are thus easier to handle and dispense into the washload, and they are more compact, hence facilitating more economical storage.
Detergent tablets are generally made by compressing or compacting a detergent powder, which includes detergent active and detergency builder. EP-A-522766 explains that difficulty has been found in providing tablets which have adequate strength when dry, yet disperse and dissolve quickly when added to wash water. The problem has proved especially difficult with compositions containing insoluble aluminosilicate as detergency builder, but the problem also arises with tablets which contain sodium tripolyphophate as the detergency builder.
This prior document teaches that at least some particles of the composition should be coated with a binder which helps to hold the tablet together and allows a tablet to be made using a lower compaction pressure. The binder can also function as a disintegrant.
U.S. Pat. No. 4,642,197 teaches that the effect of selected tablet disintegration agents in a washing additive tablet may be enhanced by the addition of not more than 7% by weight of an alkali metal salt of short-chain organic mono- or polycarboxylic acid. Sodium acetate and sodium citrate are named.
EP-A-482627 teaches that a detergent composition for compaction into tablets with improved solubility should include potassium carbonate together with nonionic surfactant.
EP-A-711827 teaches that speed of disintegration of tablets can be improved by including a highly water-soluble citrate. Tablet compositions exemplified in that document include sodium citrate dihydrate and also polyethylene glycol as an organic polymeric binder. This document also mentions that sodium acetate can be included in a composition as a lubricant to aid tableting. The trihydrate of sodium acetate is not named. The amount of lubricant is not stated, but it would be appropriate to include only a small amount.
WO 90/02165 mentions a range of materials including sodium acetate trihydrate as tableting aids, preferably used as a small percentage of the composition and preferably of fine particle size. A range of possible functions is attributed indiscriminately to these tableting aids.
Surprisingly, we have now found that the speed of disintegration of tablets can be enhanced by including sodium acetate trihydrate. This material has been found to be more effective than some other materials, including sodium citrate dihydrate, even without polymeric binder present. Moreover, we have found that sodium acetate trihydrate can be included without detriment to tablet strength. Indeed we have observed enhancements in tablet strength. Potassium acetate has also been found to be remarkably effective.
In a first aspect, the present invention provides a tablet of a compacted particulate composition wherein the tablet or a region thereof comprises first particles which contain a water-softening agent and second particles which contain sodium acetate trihydrate, potassium acetate or a mixture of them, where the second particles are separate from but mixed with the first particles.
The amount of water-softening agent will generally be at least 15% by weight of the composition. Depending on the function for which the tablets are intended the amount may range up to 90 or 93% by weight. In significant forms of this invention there is at least 15%, by weight of the composition, of a water-insoluble water softening agent.
The amount of the sodium acetate trihydrate or potassium acetate or mixture of the two is 10% by weight of the composition, often at least 13% by weight. The amount will not exceed 35% by weight of the composition and frequently will not exceed 25% or 30% by weight of the composition.
It is possible that the sodium acetate trihydrate or potassium acetate might be used jointly with sodium citrate dihydrate because sodium citrate dihydrate may function as a water-soluble water softening agent/detergency builder as well as enhancing the speed of disintegration of a tablet in water. Thus the composition of a tablet or region thereof might contain from 10% up to 20% or more of sodium acetate trihydrate or potassium acetate or a mixture of the two, accompanied by 5% to 20% by weight of sodium citrate dihydrate.
The invention includes a process for making tablets by mixing particles containing the water-softening agent with second particles containing the crystalline salt and then compacting the resulting composition to form a tablet or a region of a tablet.
We have now found, however, that when sodium acetate trihydrate, potassium acetate and mixtures thereof are handled on a commercial scale, they have a tendency to cake into inconvenient lumps even though they are simple crystalline solids. We have found that this problem, which we believe has not previously been recognised, can be reduced by applying finely divided particulate material to the exterior of the crystals. Moreover, the benefit of improved speed of disintegration is substantially retained.
Accordingly, in another aspect, this invention provides a process for the production of a tablet of a compacted particulate composition by mixing.
(i) first particles containing a water-softening agent, and
(ii) second particles containing a water-soluble crystalline salt selected from sodium acetate trihydrate, potassium acetate and mixtures thereof
and compacting the resulting mixed composition into tablets or regions of tablets, characterised by the presence of particles of another substance at the surface of the crystals of the said crystalline salt (ii) before it is mixed with the water softening agent (i).
In a second aspect this invention provides a tablet of compacted particulate composition containing a water-softening agent mixed with a crystalline salt selected from sodium citrate dihydrate, sodium acetate trihydrate, potassium acetate and mixtures thereof characterised by particles of another material at the surface of the crystals of the said crystalline salt.
The process may include a step of application of particles of material to the surface of crystals of the crystalline salt. However, this step may be carried out by the manufacturer of the salt, at the place and time of its production, prior to transport to the place where the tablets are made by mixing and compaction.
This invention utilises crystals of sodium acetate trihydrate, potassium acetate or mixture of them, preferably bearing particles of another substance at the surface of the crystals of the said salt, in a tablet of compacted particulate composition or a region thereof, to enhance the disintegration of the tablet in water.
The amount of the sodium acetate trihydrate, potassium acetate or mixture of them, is at least 10% by weight of the composition, often at least 13% by weight. It will generally not exceed 35% by weight of the composition and frequently will not exceed 25% or 30% by weight of the composition.
Although potassium acetate is very effective, it is hygroscopic. We have found it easier to use sodium acetate trihydrate which is therefore the material of preference. If a mixture of these materials is used, it is preferred that sodium acetate trihydrate provides at least 5% by weight of the composition which is compacted into a tablet or region of a tablet.
It is strongly preferred that the potassium acetate, sodium acetate trihydrate and/or mixture thereof have a mean particle size of about 250 xcexcm, preferably above 300 xcexcm (0.3 mm), better above 500 xcexcm, (0.5 mm) to facilitate flow and handling of the particulate composition prior to and during compaction. The particle size will probably have a mean value less than 2 mm, preferably less than 1 mm. Poor powder flow is disadvantageous, inter alia, in that it leads to irregular filling of dies and inconsistent tablet weight and strength.
If another material is present at the surface of the crystals it suitably has a smaller particle size than the crystals. The mean particle size of this material may be no more than 180 xcexcm or 100 xcexcm. With some materials the mean particle size may be no more than 20 xcexcm and it may be no more than 10 xcexcm or 5 xcexcm, especially if it is water-insoluble. Thus the material on the surface of the crystals may have a mean particle size which is not more than one tenth or one thirtieth the mean size of the crystals.
A number of substances have been found suitable for application to the surface of particles of the crystalline salt. Materials which have found to be suitable include alkali metal carbonate and bicarbonates, sodium aluminosilicates and particles of polyethylene glycol.
Particles of sodium aluminosilicate are particularly preferred because they function as a water-softening agent when the composition is used.
Water-softening Agent
It is particularly envisaged that this invention will be applied to tablets containing water-insoluble water softening agent, notably alkali-metal aluminosilicate. However, it could be applied in tablets containing a soluble water-softening agent such as a condensed phosphate. It could be applied in tablets containing both soluble and insoluble water softening agentsxe2x80x94as might be used in countries where a restricted quantity of phosphate detergency builder is permitted.
It is very well known that water-insoluble alkali metal aluminosilicates can function to soften water, removing calcium ions and to a lesser extent magnesium ions by ion exchange. Aluminosilicates have become strongly favoured as environmentally acceptable detergency builders.
Alkali metal (preferably sodium) aluminosilicates used in tablets of the present invention may be either crystalline, amorphous or a mixture of the two. Such aluminosilicates generally have a calcium ion exchange capacity of at least 50 mg CaO per gram of aluminosilicate, comply with a general formula:
0.8-1.5Na2O.Al2O3 .0.8-6SiO2 
and incorporate some water. Preferred sodium aluminosilicates within the above formula contain 1.5-3.5 SiO2 units. Both amorphous and crystalline aluminosilicates can be prepared by reaction between sodium silicate and sodium aluminate, as amply described in the literature.
Suitable cyrstalline sodium aluminosilicate ion-exchange detergency builders are described, for example, in GB 1429143 (Procter and Gamble). The preferred sodium aluminosilicates of this type are the well known commercially available zeolites A and X, and mixtures thereof. Also of interest is the novel zeolite P described and claimed in EP 384070 (Unilever).
Another category of water-insoluble material which can function as a water-softening agent and detergency builder is the layered sodium silicate builders disclosed in U.S. Pat. No. 4464839 and U.S. Pat. No. 4820439 and also referred to in EP-A-551375.
These materials are defined in U.S. Pat. No. 4820439 as being crystalline layered sodium silicate of the general formula
NaMSixO2x-1 .YH2O 
where
M denotes sodium or hydrogen,
x is from 1.9 to 4 and y is from 0 to 20.
Quoted literature references describing the preparation of such materials include Glastechn. Ber. 37, 194-200 (1964), Zeitschrift fxc3xcr Kristallogr. 129, 396-404 (1969), Bull. Soc. Franc. Min. Crist., 95, 371-382 (1972) and Amer. Mineral, 62 763-771 (1977). These materials also function to remove calcium and magnesium ions from water.
It is customary to use a water-soluble builder (water-softening agent) jointly with aluminosilicate, to enhance water-softening efficacy. Such water-soluble co-builders are generally used in an amount which is not greater than the amount of aluminosilicate, often less than half the amount of aluminosilicate. Water-soluble builders may be organic or inorganic. Inorganic builders that may be present include alkali metal (generally sodium) carbonate; while organic builders include polycarboxylate polymers; such as polyacrylates, acrylic/maleic copolymers, and acrylic phosphonates, monomeric polycarboxylates such as citrates, gluconates, oxydisuccinates, glycerol mono- di- and trisuccinates, carboxymethyloxysuccinates, carboxymethyloxymalonates, dipicolinates and hydroxyethyliminodiacetates.
Especially preferred supplementary builders are polycarboxylate polymers, more especially polyacrylates and acrylic/maleic copolymers, and monomeric polycarboxylates, more especially citric acid and its salts.
If a tablet contains only soluble water-softening agent, this may well be sodium tripolyphosphate, which is widely used as a detergency builder in some countries.
When using aluminosilicate or other insoluble detergency builder/water-softening agent it is often a commercial or legislative requirement to avoid phosphates. Some tablet compositions of the invention do not contain more than 5 wt % of inorganic phosphate builders, and are desirably substantially free of phosphate builders. However, tableted compositions containing some phosphate builder are also within the broad scope of the invention. In particular, a tablet or region thereof may contain at least 15 wt % insoluble water softening agent, with phosphate or other water-soluble builder in addition.
As mentioned above, compositions of this invention may be embodied as detergent compositions for use in fabric washing, in which case the composition will generally contain from 15 to 60% by weight of detergency builder, notably water-insoluble aluminosilicate, together with 5 to 50% by weight of one or more detergent-active compounds. Such a composition may well contain from 0.5 to 15% by weight of a supplementary builder, notably polycarboxylate, and also other detergency ingredients.
Tablets for use in fabric washing may in particular be used when washing in an automatic washing machine and will disintegrate and be consumed in a single machine cycle.
Another possibility is that the invention may be embodied in tablets whose principal or sole function is that of removing water hardness. In such tablets the water-softening agents, especially water-insoluble aluminosilicate, may provide from 50 to 98% of the tablet composition. A water-soluble supplementary builder may well be included, for instance in an amount from 2% to 30 wt % of the composition.
Water-softening tablets embodying this invention may include some detergent active. Notably, water-softening tablets may include nonionic surfactant which can act as a lubricant during tablet manufacture and as a low foaming detergent during use. The amount may be small, e.g. from 0.2 or 0.5% by weight of the composition up to 3% or 5% by weight.
Detergent Tablets
Tablets for use in fabric washing will generally contain from 5% to 50% by weight of detergent active, preferably from 5% or 9 wt % up to 40% or 50 wt %. Detergent-active material present may be anionic (soap or non-soap), cationic, zwitterionic, amphoteric, nonionic or any combination of these.
Anionic detergent-active compounds may be present in an amount of from 0.5 to 40 wt %, preferably from 2% or 4% to 30% or 40 wt %.
Synthetic (i.e. non-soap) anionic surfactants are well known to those skilled in the art. Examples include alkylbenzene sulphonates, particularly sodium linear alkylbenzene sulphonates having an alkyl chain length of C8-C15; olefin sulphonates; alkane sulphonates; dialkyl sulphosuccinates; and fatty acid ester sulphonates.
Primary alkyl sulphate having the formula
ROSO3 xe2x88x92Mxe2x88x92
in which R is an alkyl or alkenyl chain of 8 to 18 carbon atoms especially 10 to 14 carbon atoms and M is a solubilising cation, is commercially significant as an anionic detergent active. It is frequently the desired anionic detergent and may provide 75 to 100% of any anionic non-soap detergent in the composition.
In some forms of this invention the amount of non-soap anionic detergent lies in a range from 0.5 to 15 wt % of the tablet composition.
It may also be desirable to include one or more soaps of fatty acids. These are preferably sodium soaps derived from naturally occurring fatty acids, for example, the fatty acids from coconut oil, beef tallow, sunflower or hardened rapeseed oil.
Suitable nonionic detergent compounds which may be used include in particular the reaction products of compounds having a hydrophobic group and a reactive hydrogen atom, for example, aliphatic alcohols, acids, amides or alkyl phenols with alkylene oxides, especially ethylene oxide either alone or with propylene oxide.
Specific nonionic detergent compounds are alkyl (C8-22) phenol-ethylene oxide condensates, the condensation products of linear or branched aliphatic C8-20 primary or secondary alcohols with ethylene oxide, and products made by condensation of ethylene oxide with the reaction products of propylene oxide and ethylene-diamine. Other nonionic detergent compounds include alkylpolyglycosides, long-chain amine oxides, tertiary phosphine oxides, and dialkyl sulphoxides.
Especially preferred are the primary and secondary alcohol ethoxylates, especially the C9-11 and C12-15 primary and secondary alcohols ethoxylated with an average of from 5 to 20 moles of ethylene oxide per mole of alcohol.
In certain forms of this invention the amount of nonionic detergent lies in a range from 4 to 40%, better 4 or 5 to 30% by weight of the composition.
Many nonionic detergent-active compounds are liquids. These may be absorbed on a porous carrier. Preferred carriers include zeolite; zeolite granules with other materials, for example Wessalith CS (Trade Mark), Wessalith CD (Trade Mark) or Vegabond GB (Trade Mark); sodium perborate monohydrate; Burkeite (spray-dried sodium carbonate and sodium sulphate as disclosed in EP-A-221776 of Unilever); and layered sodium silicate as described in U.S. Pat. No. 4,664,839.
Bleach System
Tableted detergent compositions according to the invention may contain a bleach system. This preferably comprises one or more peroxy bleach compounds, for example, inorganic persalts or organic peroxyacids, which may be employed in conjunction with activators to improve bleaching action at low wash temperatures. If any peroxygen compound is present, the amount is likely to lie in a range from 10 to 25% by weight of the composition.
Preferred inorganic persalts are sodium perborate monohydrate and tetrahydrate, and sodium percarbonate, advantageously employed together with an activator. Bleach activators, also referred to as bleach precursors, have been widely disclosed in the art. Preferred examples include peracetic acid precursors, for example, tetraacetylethylene diamine (TAED), now in widespread commercial use in conjunction with sodium perborate; and perbenzoic acid precursors. The quaternary ammonium and phosphonium bleach activators disclosed in U.S. Pat. No. 4,751,015 and U.S. Pat. No. 4,818,426 (Lever Brothers Company) are also of interest. Another type of bleach activator which may be used, but which is not a bleach precursor, is a transition metal catalyst as disclosed in EP-A-458397, EP-A-458398 and EP-A-549272. A bleach system may also include a bleach stabiliser (heavy metal sequestrant) such as ethylenediamine tetramethylene phosphonate and diethylenetriamine pentamethylene phosphonate.
As indicated above, if a bleach is present and is a water-soluble inorganic peroxygen bleach, the amount may well be from 10% to 25% by weight of the composition.
Other Ingredients
Detergent tablets of the invention may also contain one of the detergency enzymes well known in the art for their ability to degrade and aid in the removal of various soils and stains. Suitable enzymes include the various proteases, cellulases, lipases, amylases, and mixtures thereof, which are designed to remove a variety of soils and stains from fabrics. Examples of suitable proteases are Maxatase (Trade Mark), as supplied by Gist-Brocades N.V., Delft, Holland, and Alcalase (Trade Mark), and Savinase (Trade Mark), as supplied by Novo Industri A/S, Copenhagen, Denmark. Detergency enzymes are commonly employed in the form of granules or marumes, optionally with a protective coating, in amount of from about 0.1% to about 3.0% by weight of the composition; and these granules or marumes present no problems with respect to compaction to form a tablet.
The detergent tablets of the invention may also contain a fluorescer (optical brightener), for example, Tinopal (Trade Mark) DMS or Tinopal CBS available from Ciba-Geigy AG, Basel, Switzerland. Tinopal DMS is disodium 4,4xe2x80x2bis-(2-morpholino-4-anilino-s-triazin-6-ylamino) stilbene disulphonate; and Tinopal CBS is disodium 2,2xe2x80x2-bis-(phenyl-styryl) disulphonate.
An antifoam material is advantageously included, especially if the detergent tablet is primarily intended for use in front-loading drum-type automatic washing machines. Suitable antifoam materials are usually in granular form, such as those described in EP 266863A (Unilever). Such antifoam granules typically comprise a mixture of silicone oil, petroleum jelly, hydrophobic silica and alkyl phosphate as antifoam active material, sorbed onto a porous absorbed water-soluble carbonate-based inorganic carrier material. Antifoam granules may be present in an amount up to 5% by weight of the composition.
It may also be desirable that a detergent tablet of the invention includes an amount of an alkali metal silicate, particularly sodium ortho-, meta- or preferably alkali metal silicates at levels, for example, of 0.1 to 10 wt %, may be advantageous in providing protection against the corrosion of metal parts in washing machines, besides providing some measure of building and giving processing benefits.
Further ingredients which can optionally be employed in the detergent tablet of the invention include anti-redeposition agents such as sodium carboxymethylcellulose, straight-chain polyvinyl pyrrolidone and the cellulose ethers such as methyl cellulose and ethyl hydroxyethyl cellulose, fabric-softening agents; heavy metal sequestrants such as EDTA; perfumes; colourants or coloured speckles, and tableting aids such as binders and lubricants.
The particulate mixed composition which is compacted into tablets may in principle have any bulk density. However, the present invention is especially relevant to tablets made by compacting powders of relatively high bulk density, because of their greater tendency to exhibit disintegration and dispersion problems. Such tablets have the advantage that, as compared with a tablet derived from a low bulk density powder, a given dose of composition can be presented as a smaller tablet.
Thus the starting particulate composition may suitably have a bulk density of at least 400 g/liter, preferably at least 500 g/liter, and advantageously at least 700 g/liter.
A tablet of the invention may be either homogeneous or heterogeneous. In the present specification, the term xe2x80x9chomogeneousxe2x80x9d is used to mean a tablet produced by compaction of a single particulate composition, but does not imply that all the particles of that composition will necessarily be of identical composition. Indeed it is likely that the composition will contain the sodium acetate trihydrate or potassium acetate as separate particles.
The term xe2x80x9cheterogeneousxe2x80x9d is used to mean a tablet consisting of a plurality of discrete regions, for example layers, inserts or coatings, each derived by compaction from a particulate composition and large enough to constitute from 10 to 90% of the whole tablet.
It is possible that the potassium acetate or sodium acetate trihydrate will be contained within one or more but not all such discrete regions of a heterogeneous tablet, such as a layer or an insert. The presence of such a layer or insert could assist break up of the entire tablet when placed in water.
Tableting
Tableting entails compaction of a particulate composition. A variety of tableting machinery is known, and can be used. Generally it will function by stamping a quantity of the particulate composition which is confined in a die.
Tableting may be carried out at ambient temperature or at a temperature above ambient which may allow adequate strength to be achieved with less applied pressure during compaction. In order to carry out the tableting at a temperature which is above ambient, the particulate composition is preferably supplied to the tableting machinery at an elevated temperature. This will of course supply heat to the tableting machinery, but the machinery may be heated in some other way also.
If any heat is supplied, it is envisaged that this will be supplied conventionally, such as by passing the particulate composition through an oven, rather than by any application of microwave energy. However, this invention could be utilised in a process in which the tableting step includes application of microwave energy to the composition.